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1The opening of large tracts by the ice-cutters commonly causes a pond to break up earlier; for the water, agitated by
2the wind, even in cold weather, wears away the surrounding ice. But such was not the effect on Walden that year, for she
3had soon got a thick new garment to take the place of the old. This pond never breaks up so soon as the others in this
4neighborhood, on account both of its greater depth and its having no stream passing through it to melt or wear away the
5ice. I never knew it to open in the course of a winter, not excepting that of '52-3, which gave the ponds so severe a
6trial. It commonly opens about the first of April, a week or ten days later than Flint's Pond and Fair-Haven, beginning
7to melt on the north side and in the shallower parts where it began to freeze. It indicates better than any water
8hereabouts the absolute progress of the season, being least affected by transient changes of temperature. A severe cold
9of a few days' duration in March may very much retard the opening of the former ponds, while the temperature of Walden
10increases almost uninterruptedly. A thermometer thrust into the middle of Walden on the 6th of March, 1847, stood at
1132°, or freezing point; near the shore at 33°; in the middle of Flint's Pond, the same day, at 32½°; at a dozen rods
12from the shore, in shallow water, under ice a foot thick, at 36°. This difference of three and a half degrees between
13the temperature of the deep water and the shallow in the latter pond, and the fact that a great proportion of it is
14comparatively shallow, show why it should break up so much sooner than Walden. The ice in the shallowest part was at
15this time several inches thinner than in the middle. In mid-winter the middle had been the warmest and the ice thinnest
16there. So, also, every one who has waded about the shores of the pond in summer must have perceived how much warmer the
17water is close to the shore, where only three or four inches deep, than a little distance out, and on the surface where
18it is deep, than near the bottom. In spring the sun not only exerts an influence through the increased temperature of
19the air and earth, but its heat passes through ice a foot or more thick, and is reflected from the bottom in shallow
20water, and so also warms the water and melts the under side of the ice, at the same time that it is melting it more
21directly above, making it uneven, and causing the air bubbles which it contains to extend themselves upward and downward
22until it is completely honeycombed, and at last disappears suddenly in a single spring rain. Ice has its grain as well
23as wood, and when a cake begins to rot or "comb," that is, assume the appearance of honey-comb, whatever may be its
24position, the air cells are at right angles with what was the water surface. Where there is a rock or a log rising near
25to the surface the ice over it is much thinner, and is frequently quite dissolved by this reflected heat; and I have
26been told that in the experiment at Cambridge to freeze water in a shallow wooden pond, though the cold air circulated
27underneath, and so had access to both sides, the reflection of the sun from the bottom more than counterbalanced this
28advantage. When a warm rain in the middle of the winter melts off the snow-ice from Walden, and leaves a hard dark or
29transparent ice on the middle, there will be a strip of rotten though thicker white ice, a rod or more wide, about the
30shores, created by this reflected heat. Also, as I have said, the bubbles themselves within the ice operate as
31burning-glasses to melt the ice beneath. The phenomena of the year take place every day in a pond on a small scale.
32Every morning, generally speaking, the shallow water is being warmed more rapidly than the deep, though it may not be
33made so warm after all, and every evening it is being cooled more rapidly until the morning. The day is an epitome of
34the year. The night is the winter, the morning and evening are the spring and fall, and the noon is the summer. The
35cracking and booming of the ice indicate a change of temperature. One pleasant morning after a cold night, February
3624th, 1850, having gone to Flint's Pond to spend the day, I noticed with surprise, that when I struck the ice with the
37head of my axe, it resounded like a gong for many rods around, or as if I had struck on a tight drum-head. The pond
38began to boom about an hour after sunrise, when it felt the influence of the sun's rays slanted upon it from over the
39hills; it stretched itself and yawned like a waking man with a gradually increasing tumult, which was kept up three or
40four hours. It took a short siesta at noon, and boomed once more toward night, as the sun was withdrawing his influence.
41In the right stage of the weather a pond fires its evening gun with great regularity. But in the middle of the day,
42being full of cracks, and the air also being less elastic, it had completely lost its resonance, and probably fishes and
43muskrats could not then have been stunned by a blow on it. The fishermen say that the "thundering of the pond" scares
44the fishes and prevents their biting. The pond does not thunder every evening, and I cannot tell surely when to expect
45its thundering; but though I may perceive no difference in the weather, it does. Who would have suspected so large and
46cold and thick-skinned a thing to be so sensitive? Yet it has its law to which it thunders obedience when it should as
47surely as the buds expand in the spring. The earth is all alive and covered with papillæ. The largest pond is as
48sensitive to atmospheric changes as the globule of mercury in its tube. One attraction in coming to the woods to live
49was that I should have leisure and opportunity to see the spring come in. The ice in the pond at length begins to be
50honey-combed, and I can set my heel in it as I walk. Fogs and rains and warmer suns are gradually melting the snow; the
51days have grown sensibly longer; and I see how I shall get through the winter without adding to my wood-pile, for large
52fires are no longer necessary. I am on the alert for the first signs of spring, to hear the chance note of some arriving
53bird, or the striped squirrel's chirp, for his stores must be now nearly exhausted, or see the woodchuck venture out of
54his winter quarters. On the 13th of March, after I had heard the bluebird, song-sparrow, and red-wing, the ice was still
55nearly a foot thick. As the weather grew warmer it was not sensibly worn away by the water, nor broken up and floated
56off as in rivers, but, though it was completely melted for half a rod in width about the shore, the middle was merely
57honey-combed and saturated with water, so that you could put your foot through it when six inches thick; but by the next
58day evening, perhaps, after a warm rain followed by fog, it would have wholly disappeared, all gone off with the fog,
59spirited away. One year I went across the middle only five days before it disappeared entirely. In 1845 Walden was first
60completely open on the 1st of April; in '46, the 25th of March; in '47, the 8th of April; in '51, the 28th of March; in
61'52, the 18th of April; in '53, the 23d of March; in '54, about the 7th of April.
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